Replace modular_inverse with built-in recip

[b13decker]

Note, we will need to add the constraint prime n to div_mod if we do this.

suggestion: If you're not trying to match a spec, you can also call the built-in recip function here.

Originally posted by @marsella in #23 (comment)

[b13decker]

I'm going to hold off on this one until the rest of the Cryptol work is done, since this is just a nice to have.